Method for preparing alkyl carboxylic acid ester and apparatus for preparing alkyl carboxylic acid ester

ABSTRACT

In methods for preparing an alkyl carboxylic acid ester according to an embodiment, a first reaction product is produced by reaction of a carboxylic acid stream and a first alcohol stream in a preliminary reactor. A second reaction product is produced by at least partially removing alcohol and water from the first reaction product. The second reaction product is esterified by reaction with a second alcohol stream in a main reactor. An alkyl carboxylic acid ester is recovered from the main reactor.

CROSS REFERENCE TO RELATED APPLICATIONS AND CLAIM OF PRIORITY

This application is a divisional application of application Ser. No.17/296,083, filed on May 21, 2021, which is a National Stage entry fromInternational Application No. PCT/KR2019/016270, filed on Nov. 25, 2019,which claims priority to the benefit of Korean Patent Application No.10-2018-0146734 filed in the Korean Intellectual Property Office on Nov.23, 2018, the entire contents of which are incorporated herein byreference.

BACKGROUND 1. Technical Field

The present invention relates to a method for preparing an alkylcarboxylic acid ester and an apparatus for preparing an alkyl carboxylicacid ester. More particularly, the present invention relates to a methodfor preparing an alkyl carboxylic acid ester through the esterificationreaction of an alcohol and a carboxylic acid and an apparatus forpreparing an alkyl carboxylic acid ester.

2. Background Art

For example, alkyl carboxylic acid esters such as ethyl lactate arewidely used as solvents of all kinds of polishing solutions, etchingsolutions, photosensitive resin compositions, resist compositions, etc.,in the manufacturing process of semiconductors or manufacturing processof displays.

In case of the manufacturing process of semiconductors, requiring thedegree of precision of tens or several nano scale, all components of acomposition are required to have high purity, and if a small quantity ofimpurities are included, the reliability of the whole manufacturingprocess of semiconductors may be deteriorated. Accordingly, the secureof ethyl lactate with high purity, used in a fine process is required.

Ethyl lactate may be obtained through, for example, the esterificationreaction of lactic acid and ethanol. Lactic acid may be obtained bybio-based synthesis through fermentation reaction using microorganism.

In this case, the raw material of lactic acid may include impuritiesderived from microorganism, fermentation raw materials, etc., andaccordingly, the purity and selectivity of ethyl lactate may be reduced.In addition, the purity and selectivity of ethyl lactate may also bereduced by impurities through the self-reaction of lactic acid.

Accordingly, for the synthesis process of ethyl lactate with high purityand high selectivity, the reactor is also required to be designed tohave high selectivity and high capacity for esterification reaction, andthe use of ethanol with high purity is required. In this case, the loadof a reactor may be excessively increased, and high cost may beconsumed.

For example, in Korean Laid-open Patent Publication No. 2005-0084179, acontinuous-type method for preparing ethyl lactate is disclosed, but asdescribed above, research on design for an esterification reactionprocess, by which impurities are removed from lactic acid, and economicfeasibility is improved, is required.

SUMMARY

A task of the present invention is to provide a method for preparing analkyl carboxylic acid ester having excellent purity and selectivity.

Another task of the present invention is to provide an apparatus forpreparing an alkyl carboxylic acid ester having excellent purity andselectivity.

1. A method for preparing an alkyl carboxylic acid ester, the methodcomprising: reacting a carboxylic acid stream and a first alcohol streamin a preliminary reactor to produce a first reaction product; at leastpartially removing alcohol and water from the first reaction product toproduce a second reaction product; esterifying by reacting the secondreaction product with a second alcohol stream in a main reactor; andrecovering an alkyl carboxylic acid ester from the main reactor.

2. The method for preparing an alkyl carboxylic acid ester according toabove 1, wherein the at least partial removing of the alcohol and wateris performed through a distillation column.

3. The method for preparing an alkyl carboxylic acid ester according toabove 1, wherein the production of the first reaction product in thepreliminary reactor comprises converting a portion of the carboxylicacid stream into an ester.

4. The method for preparing an alkyl carboxylic acid ester according toabove 1, wherein the esterifying in the main reactor comprisesconverting an unreacted carboxylic acid stream into an ester in thepreliminary reactor.

5. The method for preparing an alkyl carboxylic acid ester according toabove 1, wherein an ester conversion in the preliminary reactor is 50 to80%, and an ester conversion in the main reactor is 95% or more.

6. The method for preparing an alkyl carboxylic acid ester according toabove 2, wherein the carboxylic acid stream comprises lactic acid, thefirst alcohol stream and the second alcohol stream comprise ethanol, andthe alkyl carboxylic acid ester comprises ethyl lactate.

7. The method for preparing an alkyl carboxylic acid ester according toabove 6, wherein ethanol and water are at least partially separated andremoved from the top of the distillation column, and partially convertedethyl lactate and lactic acid are discharged from the bottom of thedistillation column.

8. The method for preparing an alkyl carboxylic acid ester according toabove 6, wherein the reaction in the preliminary reactor furthercomprises hydrolysis of an agglomerate comprising a dimer, a trimer oran oligomer produced from the lactic acid.

9. The method for preparing an alkyl carboxylic acid ester according toabove 1, wherein the first alcohol stream has lower purity than thesecond alcohol stream.

10. The method for preparing an alkyl carboxylic acid ester according toabove 2, further comprising refluxing alcohol removed from the firstreaction product in the distillation column into the first alcoholstream.

11. The method for preparing an alkyl carboxylic acid ester according toabove 1, further comprising refluxing remaining alcohol after thereaction in the main reactor into the first alcohol stream.

12. An apparatus for preparing an alkyl carboxylic acid ester,comprising: a preliminary reactor for partial esterification throughreaction of a carboxylic acid with an alcohol; a separator for receivinga reaction product from the preliminary reactor and at least partiallyseparating water and alcohol; a main reactor for receiving a reactionproduct from which water and alcohol are separated from a distillationcolumn and esterifying an unreacted carboxylic acid; and a separationand purification unit for collecting an alkyl carboxylic acid esterproduced from the main reactor.

13. The apparatus for preparing an alkyl carboxylic acid ester accordingto above 12, wherein the separator is a distillation column.

14. The apparatus for preparing an alkyl carboxylic acid ester accordingto above 12, further comprising: a carboxylic acid supply passage and afirst alcohol supply passage, connected with the preliminary reactor;and a second alcohol supply passage connected with the main reactor.

15. The apparatus for preparing an alkyl carboxylic acid ester accordingto above 13, further comprising a reflux unit for recovering water andalcohol separated from the distillation column.

According to the preparation method and the preparation apparatus of analkyl carboxylic acid ester according to embodiments of the presentinvention, for example, a preliminary reactor may be disposed at thefront of a main reactor including a reactive distillation reactor, andpreliminary conversion or partial conversion of an alcohol and acarboxylic acid may be performed. Accordingly, the degradation ofselectivity by the overload in the main reactor may be prevented, andthe efficiency of a process at the latter part may be improved.

In addition, through hydrolysis in the preliminary reactor, the purityand selectivity of an ester product may be improved by decomposing anagglomerate such as a dimer, a trimer, and an oligomer, produced fromlactic acid and by removing impurities in advance.

In addition, by performing partial conversion in the preliminaryreactor, the purity of an alcohol used may be relatively reduced, andeconomic feasibility of a process may be additionally improved.

According to exemplary embodiments, water and alcohol included in afirst reaction product after performing partial esterification reactionin the preliminary reactor may be at least partially removed through adistillation column. Accordingly, though the preparation reaction of analkyl carboxylic acid ester by the preliminary reactor and the mainreactor is repeatedly performed, the excessive injection of water, etc.,produced in the preliminary reactor into the main reactor may beprevented, and the damage of an acid catalyst, etc., carried in the mainreactor may be prevented. In addition, a reaction area in the mainreactor may increase, and the whole process yield and stability of analkyl carboxylic acid ester process may be improved.

In addition, after removing alcohol with low purity, which has beeninjected in the preliminary reactor but not participated in partialesterification reaction, together with water through the distillationcolumn, an alcohol with high purity may be directly injected into themain reactor. Accordingly, the concentration of an alcohol in thereaction product increases, the equilibrium of the preparation reactionof ethyl lactate moves toward a product direction, and the conversion oflactic acid and the selectivity of ethyl lactate may be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart for explaining a preparation apparatus and apreparation method of an alkyl carboxylic acid ester according toexemplary embodiments.

FIG. 2 is a cross-sectional view showing the schematic structure andoperation of a preliminary reactor according to exemplary embodiments.

FIG. 3 is a cross-sectional view showing the schematic structure andoperation of a distillation column according to exemplary embodiments.

FIG. 4 is a cross-sectional view showing the schematic structure andoperation of a main reactor according to exemplary embodiments.

FIG. 5 is a graph showing the conversion of lactic acid and theselectivity of ethyl lactate in accordance with the operation time of apreparation process of ethyl lactate according to an exemplaryembodiment.

FIG. 6 is a graph showing the conversion of lactic acid and theselectivity of ethyl lactate in accordance with the operation time of apreparation process of ethyl lactate according to an exemplarycomparative embodiment.

DETAILED DESCRIPTION

Hereinafter, preferred embodiments of the present invention will besuggested, but the embodiments are only illustrations of the presentinvention and do not limit the attached claims. It is apparent that aperson skilled in the art may make various changes and modifications onthe embodiments within the scope of the present invention and technicalspirit, and such modifications and changes are definitely included inthe attached claims.

The term “alkyl carboxylic acid ester” used in the present disclosurerefers to the esterification reaction product of a carboxylic acid andan alkyl alcohol. According to exemplary embodiments of the presentinvention, lactic acid may be used as the carboxylic acid, and ethanolmay be used as the alkyl alcohol. In this case, ethyl lactate may beobtained as the alkyl carboxylic acid ester.

However, the scope of the present invention is not always limited to thepreparation method and preparation apparatus of ethyl lactate, and maybe expanded to a method and apparatus for esterifying diverse carboxylicacids and alkyl alcohols.

FIG. 1 is a flowchart for explaining a preparation apparatus and apreparation method of an alkyl carboxylic acid ester according toexemplary embodiments of the present invention. FIG. 1 illustrates apreparation apparatus, and a preparation method of ethyl lactate whereinlactic acid is used as a carboxylic acid, and ethanol is used as analkyl alcohol.

Referring to FIG. 1 , the apparatus for preparing an alkyl carboxylicacid ester according to exemplary embodiments may include a preliminaryreactor (100), a distillation column (200), a main reactor (300), areflux unit (400) and a separation and purification unit (500).

The preliminary reactor (100) may be connected with a carboxylic acidsupply passage (70) and a first alcohol supply passage (90). Through thecarboxylic acid supply passage (70), a lactic acid stream may besupplied into the preliminary reactor (100) (for example, step S11).Through the first alcohol supply passage (90), a first ethanol streammay be supplied into the preliminary reactor (100) (for example, stepS13). The supply of the lactic acid stream and the supply of the firstethanol stream may be performed at the same time or in order.

The first ethanol stream may have lower purity than a second ethanolstream which will be explained later. In some embodiments, the purity ofthe first ethanol stream may be about 80 to 97%. In an embodiment, thepurity of the first ethanol stream may be about 80 to 95%. In anembodiment, the purity of the first ethanol stream may be about 80 to90%.

The term “purity” used in this disclosure may mean wt % of a targetmaterial based on the total weight. For example, ethanol with purity of80% may mean a mixture of 20 wt % of water and 80 wt % of ethanol.

Lactic acid may be a fermentation product using microorganism. Forexample, lactic acid may be obtained by saccharifying starch, sugar,cellulose, algae, other organic products and then, fermenting thesaccharified product through fermentable lactic acid by microorganism.

Accordingly, the lactic acid stream supplied to the preliminary reactor(101) may include various bio by-products including the microorganism,proteins, cells, minerals, etc.

According to exemplary embodiments, the preliminary reactor (100) mayinclude a guard reactor. The guard reactor may include a solid acidcatalyst bed, and in an embodiment, the solid acid catalyst may includesilica, zeolite, and amberlyst-based catalysts.

In the preliminary reactor (100), preliminary esterification reaction ofethanol included in the first ethanol stream and lactic acid included inthe lactic acid stream may be performed (for example, step S20). In someembodiments, the temperature of the preliminary esterification reactionmay be about 70 to 110° C., preferably, about 80 to 100° C.

In the preliminary reactor (100), lactic acid may be partially convertedinto ethyl lactate by the preliminary esterification. According toexemplary embodiments, partial conversion in the preliminary reactor(100) may be about 50 to 80%.

If the partial conversion in the preliminary reactor (100) is less thanabout 50%, reaction load in the main reactor (200) which will beexplained later, increases excessively, and the acquisition of ethyllactate with desired selectivity and purity may become difficult. If thepartial conversion is greater than about 80%, esterification in thepreliminary reactor (100) may increase excessively, and overall processefficiency may be degraded.

In some embodiments, hydrolysis may be performed in the preliminaryreactor (100) at the same time. Hydrolysis may be carried out by watercontained in the first ethanol stream and the lactic acid stream, andmay be carried out by water produced according to the preliminaryesterification reaction in the preliminary reactor.

According to exemplary embodiments, a lactic acid agglomerate may bedecomposed by the hydrolysis. The lactic acid agglomerate may includethe dimer, trimer, or oligomer of lactic acid molecules. If a largeamount of a lactic acid agglomerate is included in the lactic acidstream, the selectivity of ethyl lactate may be reduced, the load in themain reactor (200) or separation and purification unit (500) mayincrease, and overall process efficiency may be reduced.

In an embodiment, through the solid acid catalyst included in thepreliminary reactor (100) for promoting the partial esterification, thehydrolysis in the preliminary reactor (100) may be promoted at the sametime through the solid acid catalyst.

According to exemplary embodiments of the present invention, togetherwith the preliminary esterification or partial conversion, thehydrolysis of a lactic acid agglomerate is carried out in thepreliminary reactor (100), selectivity in the main reactor (300) or theseparation and purification unit (500) may increase, and the size of theseparation and purification unit (500) may be reduced.

After the partial conversion in the preliminary reactor (100), a stream(for example, a first reaction product) may be supplied to adistillation column (200) through a first reaction product passage(110). The stream after the partial conversion (for example, the firstreaction product) may include partially converted ethyl lactate,unreacted lactic acid in the lactic acid stream, unreacted ethanol inthe first ethanol stream and extra water.

In the distillation column (200), the unreacted ethanol and extra waterincluded in the first reaction product which is supplied from thepreliminary reactor (100) through the first reaction product passage(110) may be at least partially removed (for example, step S30).

In some embodiments, the removal or separation of ethanol and waterthrough the distillation column (200) may be performed so that thecontents of ethanol and water in a second reaction product supplied tothe main reactor (300) may be about 1 wt % or less, respectively.

For example, the distillation column (200) may include a vacuumdistillation column or a low-pressure distillation column. Thetemperature and pressure of the distillation column (200) may becontrolled so that water and ethanol may be selectively distilled, forexample, to conditions of a pressure of about 0.1 to 1 bar and atemperature of about 90 to 110° C.

For example, ethanol and water may be separated and removed from the topof the distillation column (200), and partially converted ethyl lactateand lactic acid may be discharged from the bottom of the distillationcolumn (200). For example, ethanol and water have lower boiling pointsthan the partially converted ethyl lactate and lactic acid, and thus,may be removed from the top of the distillation column (200).

As described above, a second reaction product from which ethanol andwater are at least partially removed may be discharged from thedistillation column (200) and supplied to the main reactor (300). Thesecond reaction product includes partially converted ethyl lactate andunreacted lactic acid in the preliminary reactor (100) and may includeextra or small amounts of ethanol and water. As described above, in anembodiment, the second reaction product may include 1 wt % or less ofethanol and water, respectively.

Since water produced as by-products of the partial esterificationreaction in the preliminary reactor (100) is removed through thedistillation column (200), the damage of a catalyst in the main reactor(300) may be prevented. In addition, the hindering of the reaction siteof the main reactor (300) by water may be prevented, and a sufficientesterification reaction area may be secured.

According to exemplary embodiments, the main reactor (300) may include asingle reactive distillation column. Since the partial esterification iscarried out in the preliminary reactor (100), the number of columns ofthe main reactor (300), or a process load may be reduced. In someembodiments, the main reactor (300) may include multiple reactivedistillation columns connected in series or in parallel.

The reactive distillation column may be charged with, for example, amedium carrying a catalyst such as a solid acid catalyst. For example,an ion exchange resin including a sulfonic acid group may be charged inthe reactive distillation column. The ion exchange resin may include,for example, polysiloxane, polystyrene, polydivinylbenzene, etc. The ionexchange resin may be surface treated with, for example, a metal such aszirconium and titanium, or oxides thereof.

In the main reactor (300), unreacted lactic acid included in a streamafter the partial conversion (for example, a first reaction product) mayundergo esterification reaction with ethanol to be converted into ethyllactate (for example, step S50).

According to exemplary embodiments, ethyl lactate conversion of about95% or more may be obtained in the main reactor (300). In an embodiment,ethyl lactate conversion of about 98% or more may be obtained in themain reactor (300), preferably, ethyl lactate conversion ofsubstantially 100% may be obtained.

In some embodiments, the temperature of the esterification reaction inthe main reactor (300) may be about 70 to 110° C., preferably, 80 to100° C.

In some embodiments, a second ethanol stream may be supplied into themain reactor (300) (for example, step S40). For example, the secondethanol stream may be supplied to the main reactor (300) through aseparately connected second alcohol supply passage (150).

The second ethanol stream may have higher purity than the first ethanolstream introduced into the preliminary reactor (100), and as describedabove, a portion of lactic acid may be partially converted using a firstethanol stream with relatively lower purity in the preliminary reactor(100). By partially converting a portion of lactic acid using the firstethanol stream with relatively lower purity in the preliminary reactor(100), the amount used of costly ethanol with high purity may be reducedwhile maintaining the desired selectivity of ethyl lactate, and economicfeasibility and efficiency of overall process may be improved.

In exemplary embodiments, the purity of the second ethanol stream may beabout 95% or more, preferably, about 99% or more. By increasing thepurity of ethanol directly introduced into the main reactor (300), theequilibrium of the preparation reaction of ethyl lactate moves toward aproduct, and the selectivity of ethyl lactate obtained may be improved.

In addition, since unreacted first ethanol stream with low purity amongthe first reaction product produced from the preliminary reactor (100)is removed together with water through the distillation column (200),the selectivity of esterification reaction through the second ethanolstream with high purity, which is directly introduced into the mainreactor (300) may be improved, and the equilibrium of esterificationreaction may be promoted toward a product direction. Accordingly, theconversion of lactic acid and the selectivity of ethyl lactate may beadditionally improved.

For example, from the bottom of the main reactor (300), a stream aftermain reaction (for example, a third reaction product) may be introducedthrough a third reaction product passage (220) to the separation andpurification unit (500).

As described above, by performing partial esterification by continuouslysupplying ethanol with relatively low purity through a reflux unit tothe preliminary reactor (100), and separately supplying ethanol withhigh purity to the main reactor (300), the economic feasibility andefficiency of a whole process may be improved, and the selectivity ofethyl lactate and the conversion of lactic acid may be improved.

In the separation and purification unit (500), a process forconcentrating and/or collecting a target ester from the stream aftermain reaction (for example, a third reaction product) produced in themain reactor (300) may be performed (for example, step S60). Accordingto exemplary embodiments, the separation and purification unit (500) maybe positioned at the rear of the main reactor (300) and may concentrateand/or collect the target ester.

In some embodiments, the separation and purification unit (500) mayinclude a distillation unit (for example, vacuum distillation column).As described above, from a third reaction product passage (220)connected with the bottom of the main reactor (300), the product aftermain reaction (for example, third reaction product) may be supplied tothe distillation unit.

The product after main reaction (for example, the third reactionproduct) may include ethyl lactate produced from the main reactor (300),remaining ethanol and by-products. For example, the by-products mayinclude an agglomerate including a dimer, trimer or oligomer producedfrom lactic acid, bio-derived residues, etc. As described above, theby-products may be at least partially decomposed through hydrolysis inthe preliminary reactor (100), and the amount of the by-products in theproduct after the reaction may be markedly reduced.

Among the products after the reaction, ethyl lactate having a lowboiling point may be taken out from the top of the vacuum distillationcolumn and obtained as a target ester. Among the products after thereaction, the by-products having high boiling points may be taken outfrom the bottom of the vacuum distillation column and removed.

As explained referring to FIG. 1 , agglomerates such as the dimer,trimer and oligomer of lactic acid are preliminarily decomposed orremoved through hydrolysis in the preliminary reactor (100), and theprocess load of subsequent processes may be reduced. Accordingly,additionally performed subsequent processes, for example, an esterhydrolysis and/or transesterification (TE) unit may be removed, or theirsizes may be reduced to increase process efficiency.

In some embodiments, in the distillation column (200) and/or mainreactor (300), remaining ethanol and water produced may be supplied tothe reflux unit (400). For example, as described above, ethanol andwater separated and removed from the first reaction product through thedistillation column (200) may be supplied through a first alcoholcollection passage (210) to the reflux unit (400). The remaining ethanoland water produced as the by-products of the esterification reaction inthe main reactor (300) may be supplied through a second alcoholcollection passage (230) to the reflux unit (400).

According to exemplary embodiments, the reflux unit (400) may include adistillation column or a distillation drum. Through the reflux unit(400), water may be at least partially removed, and concentrated ethanolmay be refluxed through a reflux passage (310) to the preliminaryreactor (100) to be combined with a first ethanol stream (for example,step S70). The reflux stream supplied through the reflux passage (310)may have substantially the same purity as the first ethanol stream.

Hereinafter, the method for preparing an alkyl carboxylic acid esteraccording to embodiments of the present invention will be explained indetail referring to particular experimental embodiments. Embodiments andcomparative embodiments included in the experimental embodiments areonly illustrations of the present invention but do not limit theattached claims. It is apparent that a person skilled in the art maymake various changes and modifications on the embodiments within thescope of the present invention and technical spirit, and suchmodifications and changes are definitely included in the attachedclaims.

Examples

1) Conversion into Ethyl Lactate in Preliminary Reactor

88 wt % of lactic acid (LA) and 80 wt % of ethanol (EtOH) were mixed andused as a feed of a preliminary reactor. The molar ratio of the lacticacid and the ethanol was controlled to 1:1. The lactic acid used abio-derived product produced from a fermentation process.

In a preliminary reactor having a single bed type charged with a solidacid catalyst, as shown in FIG. 2 , esterification reaction wasperformed at a temperature of about 80° C. under an air pressure toproduce a first reaction product.

The first reaction product included 32 wt % of ethanol, 32 wt % of ethyllactate, 20 wt % of water, and 16 wt % of lactic acid and otherby-products.

2) Removal of Ethanol and Water from Distillation Column

The partially converted first reaction product into ethyl lactatethrough the preliminary reactor was used as the feed of a distillationcolumn. As shown in FIG. 3 , a vacuum distillation column was used as avacuum column. In the vacuum distillation column, a distillation processwas performed under conditions of a pressure of −0.9 bar (gauge) and atemperature of 125° C.

From the top of the vacuum distillation column, water and alcohol in gasstates were exhausted, and from the bottom, a second reaction productincluding ethyl lactate, lactic acid, ethanol, and water was discharged.The second reaction product discharged from the bottom included 40 wt %of ethyl lactate, 58 wt % of lactic acid, 1 wt % of ethanol and 1 wt %of water.

3) Conversion into Ethyl Lactate in Main Reactor

The second reaction product from which water and ethanol were at leastpartially removed through the vacuum distillation column, was used asthe feed of the main reactor. A reactive distillation (RD) columncharged with a solid acid catalyst was used as a main reactor (see FIG.4 ).

The second reaction product was injected into the top of the mainreactor, and 99% ethanol was injected into the bottom of the mainreactor, respectively, and esterification reaction was completed at thetemperature of about 80° C. and under an air pressure. The volume ratioof the feed and ethanol injected was controlled to 1:0.5.

A product obtained from the bottom of the main reactor column (strippingpart, see FIG. 4 ) was cooled by a cooling apparatus, and a liquidcomposition was measured using Agilent 7890 restek RTx-VRX GC Column forthe flame ionization detectors (FIDS).

The preparation process of ethyl lactate according to an embodiment wasrepeatedly performed, and the composition of a product obtained from thebottom of the main reactor column was measured. Based on the compositiondata of the product, a graph showing the conversion of lactic acid (LA)and the selectivity of ethyl lactate (EL) in accordance with theoperation time of the preparation process of ethyl lactate is shown inFIG. 5 .

Comparative Example

The same preparation process of ethyl lactate as in the Example wasperformed except for not performing the removing process ofethanol/water in the distillation column.

The preparation process of ethyl lactate according to a comparativeembodiment was repeatedly performed, and the composition of a productobtained from the bottom of the main reactor column was measured. Basedon the composition data of the product, a graph showing the conversionof lactic acid (LA) and the selectivity of ethyl lactate (EL) inaccordance with the operation time of the preparation process of ethyllactate is shown in FIG. 6 .

Referring to FIG. 5 and FIG. 6 , the conversion of lactic acid and theselectivity of ethyl lactate were increased overall in the Example andComparative Example, undergone the preliminary reactor at the initiationstage of the reaction.

Through this, it could be found that both load and reaction efficiencyin the main reactor were improved by the hydrolysis of lactic acidagglomerates in advance through partial esterification utilizing a guardreactor prior to the main reaction.

However, in case of the Example undergone the distillation column afterthe preliminary reactor, the lactic acid conversion (LA conversion) andthe ethyl lactate selectivity (EL selectivity) in accordance with theoperation time were maintained constant, though the preparation processof ethyl lactate was repeatedly performed, when compared to theComparative Example.

Through this, it could be found that in case of removing water andethanol through the distillation column after the preliminary reactorbefore injecting the reaction product to the main reactor, the damage ofan acid catalyst carried in the main reactor was prevented, a reactionarea for converting into ethyl lactate was increased, and an overallprocess yield and stability were improved.

What is claimed is:
 1. An apparatus for preparing an alkyl carboxylicacid ester, comprising: a preliminary reactor for partial esterificationthrough reaction of a carboxylic acid with an alcohol; a separator forreceiving a reaction product from the preliminary reactor and at leastpartially separating water and alcohol; a main reactor for receiving areaction product from which water and alcohol are separated from adistillation column and esterifying an unreacted carboxylic acid; and aseparation and purification unit for collecting an alkyl carboxylic acidester produced from the main reactor.
 2. The apparatus for preparing analkyl carboxylic acid ester according to claim 1, wherein the separatoris a distillation column.
 3. The apparatus for preparing an alkylcarboxylic acid ester according to claim 1, further comprising: acarboxylic acid supply passage and a first alcohol supply passage,connected with the preliminary reactor; and a second alcohol supplypassage connected with the main reactor.
 4. The apparatus for preparingan alkyl carboxylic acid ester according to claim 2, further comprisinga reflux unit for recovering water and alcohol separated from thedistillation column.